1. Field of the Invention
The present invention relates to a speaker structure, especially relates to a improved speaker structure which can reproduce accurate recorded sound from an electric signal representing the sound by intercepting radiation into the air of resonant sound due to resonance of unit-support member such as a baffle, frame or enclosure type to fix a speaker unit.
2. Description of Related Art
Generally, in selecting a material of a unit-support member regardless of the shape or type such as baffle, frame, or enclosure type, it is required firstly to have high rigidity enough to assure the stable holding of a speaker unit, and secondly to have high inner acoustic loss enough to suppress the resonance of the unit-support member due to acoustic dynamic pressure from the diaphragm or cone of the speaker unit.
However, no material has been known which is to meet such contradictory requirements as above. As materials for unit-support member having a relatively high rigidity, there has been known such materials as metal, ceramics or glass. It has been tried often to make it into honeycomb structure to lighten the weight for reduction of its resonance time when the unit-support member be made of metal. However, such unit-support member inevitably causes a sharp "Q" of resonance. On use of glass or ceramics, it is impossible to lighten the weight and accordingly resonance time is relatively long. Therefore, a piece of resilient material such as rubber is put between the speaker unit and the unit-support member to prevent direct propagation of vibration of the diaphragm or cone of the speaker unit to the unit-support member. Due to such resilient piece, it is impossible to attain stable and stationary holding of the unit-support member and therefore a clear rising sound may not be obtained.
On the other hand, wood or plastics has been used as acoustic energy-absorbent materials for the unit-support member. These materials "Q" are flat in comparison with the above mentioned materials having a relatively high rigidity, but the unit-support member made of such material will cause heavy and dull resonant sound still because of the nature of late velocity of propagation. Reviewing above it is proved that radical resolution may not be attained by means of only selection of materials.
Any material having a rigidity despite of the degree, from plastics to fine ceramics, will cause a resonance. Such resonance of the unit-support member will inevitably cause significant distortion of reproduced sound from the speaker unit. An attempt of deadening resonance of the unit-support member with lead, rubber or the like has been adopted hitherto as mentioned above. But reproduction sound resulting the deadening method lacks vividness and accompanies heavy and dull sound. That is, such deadening method is insufficient and only have an effect of changing the frequency distortion of the sound wave radiating from the surface of the unit-support member rather than decreasing the total amount of radiating acoustic energy.
There are different proposals to improve the quality of reproduced sound by improvement in structure. For example, as seen from FIG. 5, Japanese Utility Model 58-23482(A) shows a speaker structure using double-walled enclosure as unit-support member having inner and outer walls 101 and 102 each made of metal plate, wood or plastics molding. The double-walled enclosure has a speaker unit 100 around its opening. These inner and outer walls 101 and 102 are separated by cubic spacers 104 to define a space 103 therebetween. The space 103 is filled with liquid, powdered material or any other vibration-attenuating substance. And the inner wall 101 is lined with a sound-absorbent material 105. Also, Japanese Utility Model 53-161624(A) shows a speaker structure having a double-walled enclosure filled with liquid between its inner and outer walls for the purpose of attenuating the vibration of the enclosure.
As may be understood from the above, the filling of a vibration-attenuating material between the inner and outer walls of a double-shelled enclosure is in an attempt of attenuating the vibration caused by the speaker unit in the unit-support member, and accordingly improving the frequency characteristics of sound reproduction. However, the filling of such material cannot prevent the resonance of the unit-support member at a relatively low frequency, and it is insufficient to intercept the resonant sound at a relatively low frequency. Also, as most important problem, the outer wall of the unit-support member directly affixes the speaker unit and the wall is exposed in the atmosphere, and therefore the vibration propagated to the support member from the speaker unit will cause a resonant sound and the sound radiates directly into the atmosphere. Further, they have a disadvantage of making the unit-support member structure complicated because they must be designed to be filled with a vibration-attenuating material. Thus, the structural attempt in the prior art could not bring radical resolution as same as the above by means of selection of materials.
Under the present circumstances as above, the present invention is based on the understanding that any materials to make unit-support member will inevitably cause resonance. Though it is impossible to remove the resonance of unit-support member completely by deadening method, it is possible to shield and intercept the radiation of the resonant sound from the unit-support member into an sound-intercepting wall. Accordingly, the inventor has reached to the idea that the resonant sound will not radiate into the air out of the sound-intercepting wall when the unit-support member is sealed up tightly with the sound-intercepting wall even if the resonant sound be generated and existed therein, and therefore almost of the resonant sound may be removed on auditory sensation.